Pathfinder - How to Play 8.3 - Concentration

"Concentration" and "Concentrating on a Spell" are two different things. Yes, I
realize that this is confusing. "Concentrating on a Spell" with a duration of
Concentration is a Standard action, and you can't cast other spells while concentrating
on a spell. Concentration is a type of check you make to keep from messing up the
casting of a spell when something distracts you partway through casting.

Concentration Check DCs

Situation

Concentration Check DC

Cast defensively

15 + double spell level

Injured while casting

10 + damage dealt + spell level

Continuous damage while casting

10 + 1/2 damage dealt + spell level

Affected by a non-damaging spell while casting

DC of the spell + spell level

Grappled or pinned while casting

10 + grappler's CMB + spell level

Vigorous motion while casting

10 + spell level

Violent motion while casting

15 + spell level

Extremely violent motion while casting

20 + spell level

Wind with rain or sleet while casting

5 + spell level

Wind with hail and debris while casting

10 + spell level

Weather caused by spell

see spell

Entangled while casting

15 + spell level

To cast a spell, you must concentrate (this doesn't require any actual action or
rolling; your character just needs to focus on casting spells). If something interrupts
your concentration while you're casting, you must make a Concentration check or lose
the spell. When you make a Concentration check, you roll d20 and add your Caster Level
and the spellcasting ability score modifier used to determine bonus spells of the same
type. Clerics, Druids, Oracles, and Rangers add their Wisdom modifier. Bards, Paladins,
Sorcerers, and Summoners add their Charisma modifier. Magi and Wizards add their
Intelligence modifier. The more distracting the interruption and the higher the level
of the spell you are trying to cast, the higher the DC (see the table above. If you
fail the check, you lose the spell just as if you had cast it to no effect.

Injury

If you take damage while trying to cast a spell, you must make a concentration check
with a DC equal to 10 + the damage taken + the level of the spell you're casting. If
you fail the check, you lose the spell without effect. The interrupting event strikes
during spellcasting if it comes between the time you started and the time you complete
a spell (for a spell with a casting time of 1 full round or more) or if it comes in
response to your casting the spell (such as an attack of opportunity provoked by the
spell or a contingent attack, such as a readied action).

If you are taking continuous damage, such as from an acid arrow or by standing in
a lake of lava, half the damage is considered to take place while you are casting a
spell. You must make a concentration check with a DC equal to 10 + 1/2 the damage that
the continuous source last dealt + the level of the spell you're casting. If the last
damage dealt was the last damage that the effect could deal, then the damage is over
and does not distract you.

Spell

If you are affected by a spell while attempting to cast a spell of your own, you
must make a concentration check or lose the spell you are casting. If the spell
affecting you deals damage, the DC is 10 + the damage taken + the level of the spell
you're casting.

If the spell interferes with you or distracts you in some other way, the DC is the
spell's saving throw DC + the level of the spell you're casting (Yes, this means that
the spell's level is added twice). For a spell with no
saving throw, it's the DC that the spell's saving throw would have if a save were
allowed (10 + spell level + caster's ability score).

Grappling or Pinned

Casting a spell while you have the grappled or pinned condition is difficult and
it requires a concentration check (DC 10 + the grappler's CMB + the level of the spell
you're casting). Pinned creatures can only cast spells that do not have somatic
components.

Vigorous Motion

If you are riding on a moving mount, taking a bouncy ride in a wagon, on a small
boat in rough water, belowdecks in a storm-tossed ship, or simply being jostled in a
similar fashion, you must make a concentration check (DC 10 + the level of the spell
you're casting) or lose the spell.

Violent Motion

If you are on a galloping horse, taking a very rough ride in a wagon, on a small
boat in rapids or in a storm, on deck in a storm-tossed ship, or being pitched roughly
about in a similar fashion, you must make a concentration check (DC 15 + the level of
the spell you're casting) or lose the spell. If the motion is extremely violent, such
as that caused by an earthquake, the DC is equal to 20 + the level of the spell you're
casting.

Violent Weather

You must make a concentration check if you try to cast a spell in violent weather.
If you are in a high wind carrying blinding rain or sleet, the DC is 5 + the level of
the spell you're casting. If you are in wind-driven hail, dust, or debris, the DC is
10 + the level of the spell you're casting. In either case, you lose the spell if you
fail the concentration check. If the weather is caused by a spell, use the rules as
described in the spell's description.

Casting Defensively

If you want to cast a spell without provoking any attacks of opportunity, you must
make a concentration check (DC 15 + double the level of the spell you're casting) to
succeed. You lose the spell if you fail.

Entangled

If you want to cast a spell while entangled in a net or by a tanglefoot bag or
while you're affected by a spell with similar effects, you must make a concentration
check to cast the spell (DC 15 + the level of the spell you're casting). You lose the
spell if you fail.

Disrupting Spells

If you are facing hostile spellcasters, it can be very advantageous to force the
spellcaster to make Concentration checks because they will may fail to cast he spell.
Read the effects above, and determine which will work best for your situation and
character.

Casting a spell with a casting time greater than 1 Swift Action provokes an attack
of opportunity, so often the best option to shut down spellcasters is to stand adjacent
to them and hit them with attacks of opportunity when they attempt to cast spells. You
can prepare an action to attack them when they cast a spell, giving you two attacks
and potentially forcing two Concentration checks. Spellcasters will usually try to
evade this by taking a Five-Foot Step away from you before they cast a spell, so
prepare you action to take a Five-Foot Step and Attack. You can include a free Five-Foot
step in prepared actions if you have made no other movement that turn.